As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and operating voltage, long cycle lifespan, and low self-discharge rate, are commercially available and widely used.
In addition, as interest in environmental problems is recently increasing, research into electric vehicles (EVs), hybrid EVs (HEVs), and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is actively underway. As a power source of EVs, HEVs, and the like, a nickel metal-hydride (Ni-MH) secondary battery is mainly used. However, research into lithium secondary batteries having high energy density, high discharge voltage and output stability is actively underway and some lithium secondary batteries are commercially available.
As cathode active materials for lithium secondary batteries, lithium-containing cobalt oxides such as LiCoO2 are mainly used. In addition thereto, use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure, and the like and lithium-containing nickel oxides such as LiNiO2 is also under consideration.
Among these cathode active materials, LiCoO2 is widely used due to excellent lifespan characteristics and charge and discharge efficiencies. However, LiCoO2 is low in safety at high temperature and expensive due to resource limitations of cobalt as a raw material and thus there is limitation in price competitiveness.
Meanwhile, lithium manganese oxides, such as LiMnO2, LiMnO4, and the like, are advantageous in that they have high thermal safety and are inexpensive and easy to synthesize. However, such lithium manganese oxides have low capacity, poor high-temperature characteristics, and low conductivity.
In addition, lithium nickel-based oxides such as LiNiO2 as a cathode active material are relatively cheap and exhibit high discharge capacity. However, crystal structures of these cathode active materials undergo rapid phase transition according to changes in volume caused during charging and discharging cycles and, when exposed to air and moisture, stability of these cathode active materials is rapidly reduced.
Thus, nickel-based lithium transition metal oxides, nickel of which is partially substituted with other transition metals such as manganese, cobalt, and the like, have recently been proposed as an alternative. These nickel-based lithium transition metal oxides substituted with other metals exhibit relatively excellent cycle characteristics and capacity characteristics. However, when such lithium transition metal oxides are used for a long period of time, cycle characteristics thereof are dramatically deteriorated and problems, such as swelling due to generation of gases in a battery, reduction in thermal safety according to low chemical stability, and the like, have yet to be addressed.
To address the problems described above, technology of using a cathode active material including a lithium transition metal oxide containing an excess of lithium is disclosed. In this case, however, capacity and output characteristics are poor, an electrolyte decomposes by pH changes due to lithium byproducts (e.g., LiOH, LiCO2, and the like) according to an excess of lithium, and resistance increases.
Therefore, there is an urgent need to develop a cathode active material that exhibits enhanced capacity and output characteristics and addresses thermal safety problems.